Generally, protective films are widely used for protecting the surface of metal products, plastic panels, or the like. Acrylic adhesive agents are widely used as adhesive agents for surface protective films due to weather resistance and transparency thereof. The acrylic adhesive agent may be a cross-linked copolymer of a (meth)acrylic acid alkyl ester monomer and a monomer including a functional group such as a carboxyl group, a hydroxyl group, or an epoxy group using a polyisocyanate compound, a melamine resin, an epoxy resin, or the like.
As objects to which the surface protective film are attached, polarizing films, plastic plates, electric appliances, automobiles, and the like need to be not only protected from adhering of contaminants such as dust but also protected from static electricity. Thus, anti-static functions as well as peel strength of a predetermined level of adhesive force or greater need to be provided to the surface protective film.
Generally, static electricity is classified into friction charging generated by friction between two different subjects and peeling charging generated by separation of a closely adhered subject. Static electricity can cause adhesion of foreign materials, such as dust, electrostatic breakdown of a device, malfunction of a measuring instrument, and fire.
Particularly, according to the widespread use of mobile computers, liquid crystal TVs, and multifunctional cell-phones, demand for liquid crystal displays continues to expand and thus inhibition of static electricity caused by integration of auxiliary devices has become a major concern.
In addition, as liquid crystal displays continue to increase in size, the size of a polarizing plate used in manufacture of liquid crystal display devices also increases. With higher processing speed, static electricity is excessively generated when a protective film attached to the polarizing plate is peeled, adversely affecting the orientation of liquid crystal in the liquid crystal display device. Thus, defects in image formation occur.
To avoid such generation of static electricity, an anti-static layer has been applied on the external surfaces of a polarizing plate or a base layer of a protective layer, but the effect is negligible and static electricity has yet to be adequately controlled. Therefore, an anti-static function needs to be added to an adhesive layer to fundamentally inhibit static electricity generation.
Conventional methods to provide an anti-static function to an adhesive layer are adding a material having a conductive component such as conductive a metal powder or a carbon particle, and adding an ionic or non-ionic material in the form of a surfactant. However, in the case of adding such an additive to impart an anti-static function, the required amount of such an additive is huge, and thus the additive is released to the surface of the adhesive agent, resulting in decrease in adhesive properties.
Korean Patent Application Publication No. 2004-0030919 describes anti-static effect by adding an organic salt in an amount of at least 5 weight %. However, according to this method, high priced organic salt has to be used in a large amount.
Korean Patent Application Publication No. 2006-0128659 discloses a method of preventing chlorosis under high temperature and high humidity conditions by adding a chelating agent capable of forming a complex with an alkali metal ion, and a metal salt. However, this method also has a problem of a decrease in low-rate peel strength due to a large amount of additives.
Japanese Laid-Open Patent Publication No. 2007-2111 discloses a technical idea of embodying ant-static properties as well as reducing the content of additives by using a copolymer of 1 to 60 parts by weight of a monomer including an alkylene oxide functional group are a side chain as a chelate of added metal ions. However, polarity of an adhesive is considerably increased and properties deteriorate with time according to durability evaluation. Thus, adhesive force is considerably increased, and anti-static properties are significantly deteriorated.